As shown in FIG. 1, a mobile communication system of the LTE (Long Term Evolution) scheme is configured so that, when a mobile station UE performs a handover from a handover source radio base station (Source eNB) to a handover target radio base station (Target eNB), the handover source radio base station (Source eNB) transmits beforehand a “UE context” of the mobile station UE to the handover target radio base station (Target eNB) during handover preparation processing (Handover Preparation).
Here, the “UE context” includes a “security context” required for “security protection (ciphering and integrity protection)” of an “access stratum (AS)”.
The “security protection” of the AS is performed using a security key KeNB commonly used by the radio base station eNB and the mobile station UE.
Note that, in handover processing, the handover source radio base station (Source eNB) is configured to transfer, to the handover target radio base station (Target eNB), not the security key KeNB itself but a parameter “KeNB*” derived from the security key KeNB or a parameter “next hop (NH)” previously acquired from a switching center MME.